The image color tone is important aspect of silver halide photographic materials, since there is an inclination for the color tone of developed silver to turn an undesirable yellowish color from the desirable bluish black or neutral black color, particularly for an observer of an image on a light table for medical care. The color tone of developed silver depends on the grain size and the grain thickness of the silver halide emulsion. The decrease in the grain thickness is accompanied with a decrease in the size and thickness of developed silver while the scattering in the blue light component is increased to result in a light having a strong yellowish color. Accordingly, as described in JP-A-60-154251 (the term "JP-A" as used herein means an unexamined Japanese patent application), 62-276539, 61-285445, 2-297539, and 3-100645, a substantially water insoluble dye is incorporated into the light-sensitive material so that the color tone of the developed silver becomes bluish black or neutral black, thus allowing adjustment of the color tone.
Further, it is known to use a mercapto compound as a color toning agent.
Meanwhile, the improvement in the covering power of a silver halide photographic material has so far been acknowledged as an important factor since it leads to the lowering of the silver/binder ratio which is necessary for rapid processing. However, an emulsion having a high covering power generally has the problem that the above silver image after a development is inclined to turn a yellowish color. This problem is notable when using a fine silver halide grains, particularly in a cubic grain having a projected area corresponding circle diameter of less than 0.4 .mu.m. The improvement in the color tone of the silver image with this fine silver halide grain is insufficient with the above-described conventional techniques.
In recent years, a laser imager in which a laser beam is scanned to record on a photographic material has been popular in the medical care field as a recording system for an output image in diagnostic equipment such as X ray CT and MRI (a nuclear magnetic resonance). The recordation of an image and development processing in a shorter time period are desired for this laser imager system to allow rapid diagnosis.
The exposing wavelength in the laser imager is 600 to 700 nm in an He-Ne laser and an AlGaP semi-conductor laser and a near infrared range to an infrared range for a semiconductor laser. A photographic material used for recording is needed to be spectrally sensitized to 600 nm or more.
Further, these photographic materials have more extensive blackened density portions compared with other light-sensitive materials for medical care, often causing the problem (hereinafter referred to as "an emulsion pick off") that the blackened portion is stripped away by a foreign substance present on the roller while processing with a roller type automatic processor. It is known that a silver/binder (as the binder, mainly gelatin) ratio may be lowered in order to improve this emulsion pick off. In order to lower the silver/binder ratio, the grain size of silver halide may be decreased to increase a covering power, but this leads to the above mentioned deterioration of the silver color tone.
Further, an increase in the covering power with a fine grain ratio is accompanied with the deterioration of the sensitivity. Accordingly, a technique for increasing the sensitivity is necessary.
Thus, a photographic material having a good silver color tone, a high sensitivity and a high covering power, and the image forming method therefor, have been desired for a recording material for a laser imager.